1. Field of the Invention
The present invention relates to an interferometer system and a method for recording an interferogram. The interferometer system and the method are preferably used to determine topological properties of an object surface from the interferogram by evaluating the recorded interferogram.
Furthermore, the invention relates to a method for providing and manufacturing an object having a target surface, wherein deviations between the target surface and an actual surface of the object are determined from an interferogram and wherein the object is provided or reworked dependent upon such deviations.
2. Brief Description of Related Art
Usually, interferometer systems are used, among others, to determine topological properties of an object surface. To this end, for example, a known reference surface and an object surface to be measured are illuminated with coherent radiation, and an object wave field reflected from the object surface and a reference wave field reflected from the reference surface are superimposed on e.g. a screen such that an interference pattern is generated thereon. From the interference pattern a difference between the optical paths from the reference surface to the screen and from the object surface to the screen may be determined position-dependently. From such differences topological differences between the object surface and the reference surface may then be determined.
Two techniques are commonly applied to determine such path differences with an interferometer system:
A first approach is the so-called fringe pattern interferometery “FPI”, wherein an optical path difference between two wavefronts is determined from positions of fringe centers of an interference pattern. In this respect, reference can be made, for example, to R. A. Jones and P. L. Kadakia, “An Automated Interferogram Technique”, Applied Optics, vol. 7, pp. 1477–1482 (1968); Zanoni, U.S. Pat. No. 4,159,522, published Jun. 26, 1979 and Zanoni, U.S. Pat. No. 4,169,980, published Oct. 2, 1979.
Another approach is the so-called phase measuring interferometry “PMI”, wherein the phase difference between the two wavefronts is calculated for each pixel of a detector from a plurality of interference patterns, said plurality of interference patterns being recorded in that different phase differences are generated therein. In this respect, reference can be made, for example, to J. H. Brunning et al., “Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses”, Applied Optics, vol. 13, pp. 2693–2703 (1974); Gallagher et al., U.S. Pat. No. 3,694,088, published Sep. 26, 1972, N. Balasubramanian, U.S. Pat. No. 4,225,240, published Sep. 30, 1980; M. Schaham, Proceedings SPIE, vol. 306, pp. 183–191 (1981); and H. Z. Hu, “Polarization heterodyne interferometry using a simple rotating analyzer. 1: Theory and error analysis”, Applied Optics, vol. 22, pp. 2052–2056 (1983).
From U.S. Pat. No. 4,594,003 there is known an interferometer system in which the frequency of the radiation source is variable so that the fringes of the interference pattern can be displaced without an optical component of the interferometer system, such as a reference surface or an object surface, having to be mechanically shifted. In said system, it is provided for a change over such a range that the fringes of the interference pattern are displaceable over a full fringe width. Four interference patterns are recorded, namely with four different frequencies of the radiation source distributed within said range. For each pixel of the detector a phase φ of the optical path difference is then calculated according to the following formula:             φ      ⁡              (                  x          ,          y                )              =          arctan      ⁡              (                                            B              ⁡                              (                0                )                                      -                          B              ⁡                              (                2                )                                                                        B              ⁡                              (                1                )                                      -                          B              ⁡                              (                3                )                                                    )              ,wherein B(0) to B(3) are the intensities of the individual images at the respective pixel.
This known method for determining path differences is less suitable if there is a further surface present in the interferometer system which likewise reflects a wave field which interferes with the wave fields reflected by the reference surface and the object surface. The resulting interference pattern is then of a particular complex nature. This situation occurs, for example, if a surface of a transparent plate with two substantially plane-parallel surfaces is to be measured.